1. Field of the Invention
The present invention is generally in the field of night vision devices of the light amplification type. More particularly, the present invention relates to an improved night vision device having an image intensifier tube (I.sup.2 T) which provides fine resolution. A microchannel plate (MCP) of the image intensifier tube has microchannels of a size which may be smaller than that achieved by conventional MCP's and which are less burdensome to manufacture. A method of making such a high-resolution MCP is set out.
2. Related Technology
Even on a night which is too dark for natural human vision, invisible infrared light is richly provided by the stars. Human vision cannot utilize this infrared night time light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night time scene. Such night vision devices generally include an objective lens which focuses invisible infrared light from the night time scene onto the transparent light-receiving face of an I.sup.2 T. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an I.sup.2 T with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of dynodes, or microchannels, with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern results. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen by another electrostatic field to produce a visible image.
Those ordinarily skilled in the pertinent arts will understand that the manufacture of conventional microchannel plates involves making a fiber pre-form which includes a round glass core of a type of glass which is etchable and is generally referred to as "core glass". This core glass is placed into the closely fitting bore of a round tube made of a type of glass which can be made electrically active as an emitter of secondary-electrons, and is generally referred to as "cladding glass". `This fiber pre-form is then heated while a vacuum is applied within the tubular cladding, causing the core and cladding to fuse together. Subsequently, this fused fiber pre-form is drawn (i.e., elongated while heated to a softened condition) to produce an elongate glass fiber of smaller dimension. This fiber is cut into lengths producing a multitude of fine-dimension glass fibers, each of which includes a core of etchable glass and a tubular cladding of electrically active glass.
A great multitude of such glass fibers each including a central fiber or "core" of core glass, and a surrounding cladding of "cladding glass," are stacked together in hexagonal bundles, are fused into a unitary body, and are then further drawn to a smaller size. Importantly, the bundles of fibers used in making a conventional microchannel plate are all composed of identical fibers, each having a core glass strand surrounded by a cladding glass sheath. A plurality of these hexagonal bundles, each including many substantially identical glass fibers, are stacked together within a heavy walled glass tube. This combination of glass tube and hexagonal bundles is commonly referred to as a boule pre-form. This boule pre-form is then fused into a unitary body in a boule-fusion furnace, producing a "boule." Next, the boule is sliced transversely into many thin plates.
Subsequently, each resulting thin plate of glass (i.e., a transverse thin slice of the boule) is subjected to an etching process to remove the core glass from each fiber of the plate. The result is a thin plate of glass having a rim provided by the heavy-walled glass tube and a central field of fine-dimension channels (i.e., microchannels) which extending between opposite faces of the plate. Conventional microchannel plates include as many as eleven million, or more, individual microchannels, each of which is approximately round. Importantly, with conventional microchannel plates, each microchannel requires the formation of a corresponding fine-dimension glass fiber, and there is a one-to-one correspondence of fibers produced from the fiber pre-form, to fibers in the hexagonal bundles, and to microchannels in the finished microchannel plate. This correspondence not only results in a great number of fibers having to be manufactured, but this great number of fibers must also be handled and positioned precisely into the hexagonal bundles during manufacturing of the microchannel plate.
A long-standing effort in night vision technology has been to provide image intensifier tubes that have fine resolution. However, because resolution of the image intensifier tubes is determined in large measure by the number and size (small size being most desirable) of the microchannels in the microchannel plate of the image intensifier tube, there exists a conflict in the conventional technology between providing fine resolution and the manufacturing burden that results from the increasing numbers of fibers that have to be made and handled for such increasingly finer-resolution microchannel plates. The numbers of the fibers required to make a microchannel plate increases, not proportionately, but geometrically with decreasing size of the microchannels in the plate. Thus, the problem of manufacturing burden grows at an expediential rate for conventional technology to provide fine-resolution microchannel plates.